Many devices used in biomedical applications require that the bulk of the device have one property and the surface of the device have a different property. For example, contact lenses may require relatively high oxygen permeability through the bulk of the lens to maintain good corneal health. However, materials that exhibit exceptionally high oxygen permeability (e.g. polysiloxanes) are typically hydrophobic and, untreated or not surface modified, will adhere to the eye. Thus a contact lens will generally have a core bulk material that is highly oxygen permeable and hydrophobic, and a surface that has been treated or coated to increase hydrophilic properties. This hydrophilic surface allows the lens to move relatively freely on the eye without adhering excessive amounts of tear lipid and protein.
A known method for modifying the hydrophilicity of a relatively hydrophobic contact lens material is through the use of a plasma treatment. Plasma treatment techniques are disclosed, for example, in PCT Publications Nos. WO 96/31793 to Nicolson et al., WO 99/57581 to Chabrecek et al., and WO 94/06485 to Chatelier et al. In the Chabrecek et al. application, photoinitiator molecules are covalently bound to the surface of the article after the article has been subjected to a plasma treatment which provides the surface with functional groups. A layer of polymerizable macromonomer is then coated onto the modified surface and heat or radiation is applied to graft polymerise the macromer to form the hydrophilic surface.
Plasma treatment processes, however, require a significant capital investment in plasma processing equipment. Moreover, plasma treatments take place in a vacuum and, thus, require that the substrate be mostly dry before exposure to the plasma. Thus, substrates, such as contact lenses, that are wet from prior hydration or extraction processes must be dried, thereby further adding to both the capital and production costs. As a result of the conditions necessary for plasma treatment, the incorporation of a plasma treatment process into an automated production process is extremely difficult.
Other methods of permanently altering the surface properties of polymeric biomaterials, such as contact lenses, have been developed. Some of these techniques include Langmuir-Blodgett deposition, controlled spin casting, chemisorptions, and vapor deposition. Examples of Langmuir-Blodgett layer systems are disclosed in U.S. Pat. Nos. 4,941,997; 4,973,429, and 5,068,318. Like plasma treatment, these techniques are not cost-effective methods that may easily be incorporated into automated production processes for making biomedical devices such as contact lenses.
A more recent technique developed for coating substrates is a layer-by-layer (“LbL”) polymer absorption process, which is described, for example, in WO 99/35520 to Winterton at al., which concerns the absorption of polyionic compounds on “inert” materials.